1. Field of the Invention
The invention relates to a motor driving device, and more particularly to a soft-cut motor driving device for preventing backflow current.
2. Description of the Related Art
As electronic components increase requirement for more and more power, more and more heat has to be accordingly dissipated. Therefore, various heat-dissipation devices have already been developed, with the most popular being motor-controlled fans.
The description of a single-phase motor is discussed hereafter, with reference to FIG. 1 and FIG. 2. FIG. 1 illustrates a schematic diagram of a typical single-phase DC motor driving device. And FIG. 2 is a signal oscillogram of a typical motor driving device. As shown in FIG. 1 and FIG. 2, the typical single-phase DC motor driving device 10 comprises a Hall sensor 12, a detecting device 14, a control circuit 16 and a full-bridge driving circuit 18. Hall sensor 12 is used to detect the rotational position of the motor rotor and generate a first sensing signal SD1 and a second sensing signal SD2. The detecting device 14 is used to generate a clock signal SCLK according to the first sensing signal SD1 and the second sensing signal SD2. The control circuit 16 is used to generate four sets of driving signals A, B, C and D according to the clock signal SCLK. The full-bridge driving circuit 18 comprises a first switch SW1, a second switch SW2, a third switch SW3, a fourth switch SW4 and an inductor L. The full-bridge driving circuit 18 is coupled to a supply voltage VCC. The first switch SW1 and the second switch SW2 are respectively controlled by one of the driving signals A and D which are generated from the control circuit 16, while the third switch SW3 and the fourth switch SW4 are respectively controlled by the driving signal C, D which are generated from the control circuit 16. One end of the inductor L is coupled to the first switch SW1 and the fourth switch SW4 at the point N1, and the other end of the inductor L is coupled to the third switch SW3 and the second SW2 at the point N2. The first switch SW1 and the second switch SW2 are turned on or off in accordance with the third switch SW3 and the fourth switch SW4. Specifically, when the first switch SW1 and the second switch SW2 are turned on and the third switch SW3 and the fourth switch SW4 are turned off, an inductor current IL would flow through the inductor L from the point N1 to the point N2. Alternatively, when the first switch SW1 and the second switch SW3 are turned off and the third switch SW3 and the fourth switch SW4 are turn on, the inductor current IL on the inductor L would flow from the point N2 to the point N1. Therefore, the rotational direction and speed of the motor may be controlled by appropriately changing the quantity and the direction of the driving current of the inductor L. The first switch SW1, the second switch SW2, the third switch SW3 and the fourth switch SW4 may be respectively composed of transistors.
When the motor rapidly switches the switches SW1˜SW4 of the full-bridge circuit 18, a high-frequency voltage pulse may occur, which increases rotating motor noise. Moreover, during the switching process, if the current through the motor is unable to be released in a short time, the inductor current IL would flow back to the supply voltage VCC and generate a voltage surge to cause the motor driving device 10 broken.
FIG. 2 is a schematic diagram illustrating a signal generated by subtracting the second sensing signal SD2 from the first sensing signal SD1, the clock signal SCLK, two driving signals SC1 and SC2 flowing through the point N1 and N2, respectively, and the inductor current IL. When the first sensing signal SD1 generated by the Hall sensor 12 is larger than the second sensing signal SD2, the signal (SD1-SD2) from the first sensing signal SD1 subtracting the second signal SD2 is positive. Since the detecting device 14 is a hysteresis comparator, there exists a time de-glitch, as label t (de-glitch) in FIG. 2 shows, when comparing the clock signal SCLK generated by the detecting device 14 with the signal (SD1-SD2) made by subtracting the second sensing signal SD2 from the first sensing signal SD1. Note that the corresponding level of the clock signal SCLK changes as the motor switches switches, which is the so-called soft-cut technology. Specifically, the clock signal SCLK will alter the switches SW1˜SW4 of the full-bridge driving circuit 18 via the control circuit 16. However, even if supported by the soft-cut technology, if the switches SW1˜SW2 complete the “soft-cut” but the direction of the inductor current IL still doesn't immediately change, the inductor current IL will backflow to the supply voltage VCC via the turned-on switches SW1˜SW4 which are coupled to the supply voltage VCC and generate voltage surge at the output end as shown in the periods (d), (e) and (f) in FIG. 2.
Therefore, important issues when developing motor driving devices is to employ the soft-cut technology to drive motors with reduced noise, and employ protective devices to prevent the current of the motor to flow back to the supply voltage VCC.